1. Field of the Invention
This invention relates to paperboard packages or cartons suitable for distributing, marketing and heating prepared food products. Such structures of this type, generally, include a coating which is mass stable below 400.degree. F., has chloroform-soluble extractives not exceeding 0.5 mg/in..sup.2 of a food contact surface when exposed to a food simulating solvent of 150.degree. F. for two hours, is flexible enough to withstand conventional scoring in a cross-direction with a 2 point male rule and 0.062 inch channel while sustaining a crack length ratio of no greater than 0.1 and exhibits resistance to blocking when stacked under a load at ambient conditions of 0.5 lbs/sq. in. or greater.
2. Description of the Related Art
To meet complex purity and performance specifications, highly specialized packaging systems have been developed for distributing, marketing and heating food for service and consumption. Many of these packaging systems are based upon a structural substrate folded from a pre-printed and die-cut bleached sulphate paperboard as described by U.S. Pat. No. 4,249,978 ('978) to T. R. Baker, entitled "Method of Forming A Heat Resistant Carton", U.S. Pat. No. 3,788,876 ('876) to D. R. Baker et al., entitled "Carton Blanks Printed With A Heat Sealable Composition And Method Thereof", and commonly assigned U.S. Pat. No. 4,930,639 ('639) to W. R. Rigby, entitled "Ovenable Food Container With Removal Lid".
To protect the paper package or carton from moisture degradation, due to direct contact with a food substance, the internal surfaces of such a carton are coated with a moisture barrier of one or more continuous films of thermoplastic resin. These films are usually applied to the paperboard web, prior to printing and cutting, as a hot, viscous, extruded curtain. Low density polyethylene (LDPE), polypropylene (PP) and polyethylene terephthalate (PET) are some of the more common thermoplastic resins used for this purpose.
Also, paperboard-based food trays may take one of several forms including a press formed tray, a molded pulp tray, a solid plastic tray or a folded tray. However, trays of the foregoing description require three separate converting operations following the manufacture of the tray web: 1) extrusion of the thermoplastic barrier coating; 2) printing of the sales graphics; and 3) die-cutting of the carton tray blank. Consolidating these operations into a single operation would offer obvious economic advantages. Moreover, relatively high coat weights are required for an extruded moisture barrier (typically from 11 to 26 pounds per 3000 ft..sup.2 of ream) since lighter coat weights usually result in an inconsistent polymer layer thickness or a layer with little or no adhesiveness to the paperboard. Consequently, a more advantageous carton tray, then, would be presented if the thermoplastic barrier could be eliminated while reducing the number of converting operations.
It is apparent from the above that there exists a need in the art for a carton tray and lid which is capable of adequately protecting the food product and avoids the use of the thermoplastic barrier, but which at the same time is capable of being constructed in a single converting operation. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.